The present invention relates to a microlithographic projection system and more particularly to such a system which provides improved working resolution and improved depth of focus.
Microlithography is widely employed in the manufacture of semiconductor integrated circuits. Typically, the image of a reticle is projected at reduced scale onto the surface of a semiconductor wafer which has been coated with a suitable photosensitive resist. Exposure and developing of the resist then allows features or regions of the wafer surface to be appropriately treated, e.g. by etching or the introduction of dopant elements.
Efforts have been ongoing to increase the complexity and sophistication of integrated circuits by reducing feature size. While the reduction of feature size has pushed the art of optical lithography towards its theoretical limits, there still exists a number of practical problems where improvement is possible. Among the practical problems which remain is the limited ability of presently available automatic focussing systems to precisely determine the height of the wafer with respect to the normally vertical axis of the projection lens. Similarly, at the very fine feature sizes being utilized, the thickness of the resist coating itself becomes appreciable. Accordingly, any advancement in the optical system which will improve depth of focus and effective working resolution is highly significant in the industry.
It is known that image contrast increases if the angular distribution of illumination is weighted towards the periphery of the illuminator numerical aperture. By this is meant that the light approaching each point on the reticle is angularly concentrated in an annular fashion. Such an angular concentration can be easily produced by introducing a stop which not only defines the diameter of the effective light source, but also includes an occluding central spot which blocks light which is proceeding on or close to the optical axis. Such an approach, however, is highly inefficient and adversely impacts system throughput, since it effectively throws away a major portion of the light available to expose the resist.
Among the several objects of the present invention are the provision of a microlithographic projection system which provides improved effective working resolution; the provision of such a system which provides improved depth of focus; the provision of such a system which effects an annular concentration of the angular distribution of the projected light; the provision of such a system which efficiently utilizes available light; the provision of such a system which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.